Vertical channel transistor and method of fabricating the same

ABSTRACT

A vertical channel transistor includes a plurality of active pillar patterns extending perpendicularly from the top surface of the substrate toward an upper part. A gate insulating layer is deposited on the side wall of the active pillar pattern and serves as an ion diffusion barrier between the pillar patterns and surrounding lower gate electrodes. The resultant pillar pattern structure is encapsulated with a metal. The resultant pillar pattern is surrounded on all sides by a specified height by a sacrificial layer of Spin-On Dielectric (SOD). The metal layer is etched-back to the height of the sacrificial layer, thus forming the lower gate electrodes. A spacer layer of an insulating mater is deposited surrounding the upper part of the pillar patterns and the sacrificial layer is removed exposing a part of the lower gate electrodes. The exposed gate electrode is etched to facilitate semiconductor integration.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims priority of Korean patent applicationnumbers 10-2007-0136439 and 10-2008-0112226, filed on Dec. 24, 2007, andNov. 12, 2008, respectively, which is incorporated herein by referencein their entirety.

BACKGROUND

One or more embodiments relate to a method of fabricating asemiconductor device, and more particularly, to a vertical channeltransistor within the semiconductor device and a method of fabricatingthe same.

Recently, the integration density of semiconductor memory devices,particularly in Dynamic Random Access Memory (DRAM), has been increasedand is now approaching a gigabit of memory. The gigabit DRAM device iscomprised of a plurality of unit cells, each unit cell requiring a pitchof 4F² (wherein F denotes the minimum feature size). Upon increasing theintegration density of the semiconductor device, the size of atransistor constituting a unit cell should be decreased.

The structure of a typical planar-type Metal Oxide Semiconductor (MOS)transistor includes a gate electrode formed on the upper part of thesemiconductor substrate and a junction formed within the substrate onboth sides of the gate electrode. Accordingly, it is difficult toachieve a planar-type MOS transistor having a unit cell of 4F² despitehaving a scaled channel length.

A vertical channel transistor structure has been proposed as a means toovercome the limitations of integrating planar-type MOS transistors.

FIG. 1 is a cross-sectional view showing the detail of a gate electrodeof a conventional vertical channel transistor.

As shown in FIG. 1, the conventional vertical channel transistorincludes an active pillar pattern 106 having a top pillar pattern 106Aand a neck pillar pattern 106B. The neck pillar pattern 106B is achannel part of the transistor, and the top pillar pattern 106A is adrain part of the transistor. A poly-silicon gate electrode 112 isformed by interposing a gate oxide layer 110 surrounding the sidewallsof the neck pillar pattern 106B. A capping layer 108 is formedencapsulating the top pillar pattern 106A and a hard mask pattern 104.

The active pillar pattern 106 is formed by isotropic and anisotropicetching of the silicon substrate 102, while using the hard mask pattern104 as an etch barrier layer. The poly-silicon gate electrode 112 isformed by depositing a layer of poly-silicon on the resultant pillarpattern structure and, then, etching-back the layer of poly-silicon,while leaving the gate electrode formed in the recesses of the nickpillar pattern 106B.

One of the problems with the conventional vertical channel transistor ofFIG. 1 is that the etching may not be properly performed when etchingthe poly-silicon for a gate patterning. Thus, poly-silicon residue mayaccumulate to form a bridge between the pillar patterns. This becomes amore serious problem with increased density of the pillar pattern due toincreased integration density.

Over-etching is another potential problem with the fabrication of thevertical channel transistor of FIG. 1. As a means for preventing thebridge generation, the poly-silicon layer may be etched too deeply,opening a lower gate oxide layer and partial etching of the substratebelow. Thus, a punch phenomenon can occur.

Furthermore, because the gate is made of poly-silicon, a high gateresistance becomes a problem as the density of the resultant structureis increased. That is, characteristics, i.e. conductivity, of thevertical channel transistor begin to degrade.

SUMMARY

One or more embodiments have been proposed in order to overcome theabove-described problems with the prior art vertical channeltransistors. One or more embodiments are directed to providing avertical channel transistor with decreased gate resistance by applying ametal to a gate.

One or more embodiments are directed to providing a vertical channeltransistor and a method of fabricating the vertical channel transistorcapable of stabilizing a following process of manufacture by forming astraight bar-type pillar pattern.

Still one or more embodiments are directed to providing a method offabricating a vertical channel transistor capable of forming a metalgate with a stabilized process of manufacture.

In accordance with one or more embodiments, there is provided a verticalchannel transistor, which includes: a plurality of active pillarpatterns extending perpendicularly from a substrate toward an upperpart; and a plurality of metal gate electrodes formed by interposinggate insulating layers on sidewalls of the active pillar patterns.

Each of the active pillar patterns may include a channel active pillarpattern unit in a lower part and a drain active pillar pattern unit inan upper part, and the metal gate electrodes may be formed surroundingsidewalls of the channel active pillar pattern units. The channel activepillar pattern unit and the drain active pillar pattern unit may have asubstantially uniform line width, and the active pillar pattern may havea straight cylindrical shape. The line width of the channel activepillar pattern unit may be formed narrower than the line width of thedrain active pillar pattern unit.

The vertical channel transistor may further include a hard mask patternsformed above an upper part of the drain active pillar pattern unit. Thevertical channel transistor may further include spacers formedsurrounding the sidewalls of the hard mask pattern and the drain activepillar pattern unit. Each of the metal gate electrodes may include abarrier metal formed in contact with the gate insulating layer. Themetal gate electrode may be made of any material selected from a groupconsisting of Ta, Ti, TiN, TiB₂, TaN and TaSiN and a combinationthereof.

In accordance with another embodiment, there is provided a method offabricating a vertical channel transistor, the method including: forminga plurality of active pillar patterns by etching a substrate; forming ametal for a gate surrounding an upper part of a resultant pillar patternstructure; forming a sacrificial layer which completely fills a spacebetween the active pillar patterns; partially etching the metal to aheight of the sacrificial layer; forming a spacer on a sidewall of anupper part of the active pillar pattern exposed by the partial etchingof the metal; removing the sacrificial layer; and performing a gateseparation by etching the metal exposed by the removal of thesacrificial layer.

The active pillar pattern may be formed into a straight cylindricalshape. The width of an upper part of the active pillar pattern may beformed wider than the width of a lower part of the active pillarpattern.

A Spin-On-Dielectric (SOD) may be used as the sacrificial layer. Themetal may include a barrier metal and an electrode metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a gate electrode of aconventional vertical channel transistor.

FIG. 2 is a cross-sectional view depicting a gate structure of avertical channel transistor in accordance with one embodiment.

FIGS. 3A to 3I are cross-sectional views illustrating a process offorming the gate of the vertical channel transistor in accordance withthe embodiment of FIG. 2.

FIG. 4 is a cross-sectional view illustrating a gate structure of avertical channel transistor in accordance with another embodiment.

DESCRIPTION OF EMBODIMENTS

In order to describe in detail such that those skilled in the art easilyimplement the spirit and scope of one or more embodiments, theembodiments will be described with reference to the accompanyingdrawings.

FIG. 2 is a cross-sectional view depicting a gate structure of avertical channel transistor in accordance with one embodiment.

The vertical channel transistor in accordance with the embodimentincludes an active pillar pattern 201 which provides an active of thetransistor. The upper part of the active pillar pattern 201 is the drainactive pillar pattern 201A of the transistor and the lower part of theactive pillar pattern 201 is the channel active pillar pattern 201B ofthe transistor. The active pillar pattern 201 has a cylindrical shapewhere the channel active pillar pattern 201B and the drain active pillarpattern 201A have substantially the same line width W. A substrate belowthe channel active pillar pattern 201B is a source region 201C.

The active pillar pattern 201 is formed by anisotropically etching asilicon substrate 200 using a hard mask pattern 202 and 203 as aprotective barrier layer. The hard mask pattern is formed by depositingan oxide layer 202 and a nitride layer 203. Alternatively, the hard maskpattern can be formed of a thin film material, such as a polymer. Thehard mask pattern can be deposited as a single-layer or multi-layeredwith different materials. The multi-layered hard mask pattern is used toovercome the problems associated with the manufacturing process, whichwill hereafter be explained in detail.

A gate oxide layer 204 is interposed between a metal gate 205 and 206and the sidewalls of the channel active pillar pattern 201B. The metalgate is formed by depositing a barrier metal 205 of TaN and an electrode206 of TiN. The barrier metal can also be omitted if desired. While theelectrically conductive TiN exemplifies the metal used as the electrode,another type of metal can be used in a single-layer or multi-layeredform. For instance, the metal can be any material selected from a groupconsisting of Ta, Ti, TiN, TiB₂, TaN and TaSiN and combinations thereof.

Accordingly, by forming the gate electrode of the vertical channeltransistor out of a metal, the resistance can be lowered, therebyimproving the characteristics of the transistor. On the sidewall of thedrain active pillar pattern 201A and extending to the sidewall of thehard mask pattern 202 and 203, a spacer 208 is formed surroundingthereon and is made of an insulating layer, such as a nitride layer.

As shown in FIG. 2, the gate oxide layer 204 extends perpendicularlyfrom the substrate to surround the drain active pillar pattern 201A andthe hard mask pattern 202 and 203. To this extent, the oxide layer 204serves as a capping layer to the drain active pillar pattern and thehard mask pattern.

Also, the cylindrical shape of the pillar pattern stabilizes thefabrication process for the gate, which will be explained in detailhereafter.

FIGS. 3A to 3I are cross-sectional views illustrating a method offabricating the improved vertical channel transistor shown in FIG. 2.

As shown in FIG. 3A, an active pillar pattern 301 is formed byanisotropically etching a substrate 300 using a hard mask pattern 302and 303 as an etching barrier. This anisotropic etch gives the pillarpattern 301 its cylindrical shape.

In the embodiment, the hard mask pattern has a multi-layered structurewhere an oxide layer 302 and a nitride layer 303 are deposited.Alternatively, the hard mask pattern can be made of other materials,such as polymer. The multi-layered hard mask pattern functions as anetching barrier for subsequent etching processes of a silicon layer, aSilicon On Dielectric (SOD), a nitride layer, a metal, etc.

As shown in FIG. 3B, an oxide layer 304 is formed, by an oxidizingprocess, encapsulating the entire exterior surface of the active pillarpattern 301 and covering the top surface of the substrate. In this case,the oxide layer 304 simultaneously functions as both a gate insulatinglayer and a capping layer. Thermal oxidization, plasma oxidization orthe like are used during formation of the oxide layer 304.

As shown in FIG. 3C, a barrier metal 305, made of TaN is deposited fullyencapsulating the resultant pillar pattern structure. The barrier metal305 prevents mutual atom diffusion between a silicon and the metal usedas the electrode and enhances a mutual adhesive strength between themetal and the silicon.

As shown in FIG. 3D, an electrode 306 made of TiN is deposited to fullyencapsulate the resultant pillar pattern structure.

As shown in FIG. 3E, a Spin-On-Dielectric (SOD) 307 is depositedsurrounding the resultant pillar pattern structure. The SOD 307functions as a sacrificial layer for lowering etching target of a gatemetal. The SOD 307 has excellent gap-fill characteristics and thus cansufficiently fill a space between the pillar patterns. Alternatively,other sacrificial layers having excellent gap-fill characteristics canalso be used. Thereafter, the SOD 307 is etched-back accordingly tosatisfy the desired height for the lower electrode. That is, the SOD 307is removed from the sidewall part of the drain active pillar pattern301A and the SOD 307 remains on the sidewall part of the channel activepillar pattern 301B.

As shown in FIG. 3F, the lower electrode 306 and the barrier metal 305are exposed by etching back the SOD 307.

As shown in FIG. 3G, a spacer 308 is formed by depositing an insulatinglayer, such as nitride, and anisotropically whole surface-etching thenitride layer without a mask. Thereby, the etched-back SOD 307 isexposed. The spacer 308 protects the upper part of the pillar pattern.

As shown in FIG. 3H, the exposed SOD 307 is removed. The removal of theSOD 307 can be performed through wet-etching.

As shown in FIG. 3I, the portions of the electrode 306 and barrier metal305 exposed by the removal of the SOD 307 are etched so that gateseparation between neighboring cells is effectively accomplished.

As above-described, after the pillar pattern is formed to a cylindricalshape, the metal gate is formed surrounding its sidewalls according tothe method of fabricating the vertical channel transistor in accordancewith the embodiment. In addition, after depositing the metal, the metalis partially etched to provide the metal gate separation.

As a result of the metal previously being partially etched, the etchtarget of the metal for the metal gate separation is as small as thepartially etched amount. Therefore, over-etching into the gate oxidelayer and the substrate can be prevented. Thus, defects in thefabricating process such as residual, bridge and punch can be prevented.That is, the fabrication process is stable.

FIG. 4 is a cross-sectional view showing a vertical channel transistorin accordance with another embodiment.

The vertical channel transistor in accordance with the embodimentincludes an active pillar pattern 401 which provides for an activetransistor. The upper part of the active pillar pattern 401 becomes adrain active pillar pattern 401A of the transistor and the lower partbecomes a channel active pillar pattern 401B of the transistor. Herein,a line width W1 of the channel active pillar pattern 401B is narrowerthan the line width W2 of the drain active pillar pattern 401A. Asubstrate below the lower part of the channel active pillar pattern 401Bis a source region 401C.

The active pillar pattern 401 is formed by etching a silicon substrate400 using a hard mask pattern 402 and 403 or the like as an etch barrierlayer. At this time, a combination of isotropic and anisotropic etchingcan be used. The hard mask pattern is a multi-layered deposition of anoxide layer 402 and a nitride layer 403. Herein, other materials, suchas a polymer, can also be used as the hard mask pattern.

A gate oxide layer 404 is interposed between the sidewall of the channelactive pillar pattern 401B and a metal gate 406. In the same manner withthe previously explained embodiment, the metal gate can include abarrier metal.

A spacer 408 is formed of an insulating layer, e.g. a nitride layer andsurrounding the sidewalls of the drain active pillar pattern 401A andthe hard mask pattern 402 and 403. The gate oxide layer 404 extendsperpendicularly from the top surface of the substrate along thesidewalls of the drain active pillar pattern 401A and the hard maskpattern. Accordingly, the oxide layer 404 serves as a capping layer tothe drain active pillar pattern 401A and the hard mask pattern.

As above-described, the active pillar pattern 401 included in thevertical channel transistor in accordance with this embodiment is notcylindrical shaped as in previous embodiments; however, these structuresare substantially equivalent. In addition, the method of fabricating thevertical channel transistor is the same with the exception of the pillarpattern formation process.

Thus, this embodiment has the same effect, i.e., the same improvements,the same characteristics and the same advantages in fabrication of thetransistor, as in the previous embodiment.

One or more embodiments can improve characteristics of the verticalchannel transistor by decreasing a gate resistance and solving a gatedepletion effect (doping depletion phenomenon of a poly-silicon). Theseimprovements result from forming a gate using a metal instead ofpoly-silicon during fabrication of the vertical channel transistor.

Also, in accordance with one or more embodiments, since a metal gate isformed on the sidewall of an active pillar pattern after the activepillar pattern is formed into a cylindrical shape, defining the metalgate is easy.

Also, in accordance with one or more embodiments, after depositing ametal, the metal is partially etched back using a sacrificial layer asan etch stop and, thereafter, a metal gate separation process (gateseparation between neighboring transistors) is carried out. Therefore,an etching through the gate oxide layer and punching the substrate canbe prevented; thus, a defect in fabrication such as generation of abridge can be prevented. That is, in accordance with one or moreembodiments, the fabrication process is stable.

While the embodiments have been described, it will be apparent to thoseskilled in the art that various changes and modifications may be made.

1. A vertical channel transistor, comprising: a plurality of activepillar patterns extending perpendicularly from a top surface of thesubstrate toward an upper part; and a plurality of metal gate electrodesformed by interposing gate insulating layers on sidewalls of the activepillar patterns.
 2. The vertical channel transistor of claim 1, whereineach of the active pillar patterns includes a channel active pillarpattern unit in a lower part and a drain active pillar pattern unit inan upper part, and the metal gate electrodes are formed surroundingsidewalls of the channel active pillar pattern units.
 3. The verticalchannel transistor of claim 2, wherein the channel active pillar patternunit and the drain active pillar pattern unit have a substantiallyuniform line width, and the active pillar pattern has a straightcylindrical shape.
 4. The vertical channel transistor of claim 2,wherein a line width of the channel active pillar pattern unit is formednarrower than a line width of the drain active pillar pattern unit. 5.The vertical channel transistor of claim 3, further comprising a hardmask pattern formed above an upper part of the drain active pillarpattern unit.
 6. The vertical channel transistor of claim 5, furthercomprising spacers formed surrounding the sidewalls of the hard maskpattern and the drain active pillar pattern unit.
 7. The verticalchannel transistor of claim 1, wherein each of the metal gate electrodesincludes a barrier metal formed in contact with the gate insulatinglayer.
 8. The vertical channel transistor of claim 1, wherein the metalgate electrode is formed of any material selected from a groupconsisting of Ta, Ti, TiN, TiB.sub.2, TaN and TaSiN and a combinationthereof. 9-14. (canceled)